The present invention relates to a method for removing fluorine from phosphatic materials. More particularly, this invention is concerned with the removal of fluorine in the form of fluoride ions from a crude phosphoric acid which has been converted to an inorganic phosphate salt solution to conveniently remove the fluorine, and then reconversion of the phosphate salt solution to a purified phosphoric acid.
The manufacture of phosphoric acid has generally been accomplished by two methods. One method is called the "electric furnace" or "thermal method", and the other method of manufacture is called the "wet process" method. Both methods of manufacture are well known in the art and described, for example, in Waggaman "Phosphoric Acid, Phosphates and Phosphatic Fertilizers", Second Edition, at pages 158 to 210, Hafner Publishing Company (1969), and also in Slack, "Phosphoric Acid", Volume 1, Part 1, at pages 5 to 54, Marcel Dekker, Inc. (1968).
Phosphoric acid manufactured by the "electric furnace" or the "thermal acid" method, or by the "wet process" method, contains fluorine largely in the form of hydrogen fluoride, fluosilicic acid or fluosilicates.
When the presence of sodium salts is not objectionable in the final product, some fluorine can be precipitated and filtered off as sodium fluosilicate by the addition of an amount of sodium carbonate sufficient to convert 50 of 75% of the acid into monosodium phosphate. However, the amount of fluorine remaining is in excess of food grade requirements.
U.S. Pat. No. 1,329,273 to Ross discloses a process for removing fluorine from phosphoric acid by the addition of either calcium chloride or sodium phosphate or both salts.
U.S. Pat. No. 2,123,785, discloses the removal of fluorine by filtering the impure phosphoric acid through beds of hydrated silica.
U.S. Pat. No. 2,132,349, to Booth et al suggests the absorption of fluorine compounds by passing concentrated phosphoric acid through a column of skeletal silica.
U.S. Pat. No. 2,169,100 to Hettrick, proposes reducing the fluorine content of concentrated phosphoric acid by blowing superheated steam through it at a temperature of 280.degree. F.
The above prior art methods for removing fluorine from phosphoric acid produced in any desired manner are generally characterized by treating the crude acid in the acid form for removal of fluorine and other impurities.
U.S. Pat. Nos. 2,262,745 and 2,417,462, disclose the use of tricalcium phosphate and trimagnesium phosphate for removing fluorine from drinking water. Additionally, an article by Adler, "Removal of Fluorine from Potable Water by Tricalcium Phosphate" in Volume 30 of Industrial & Engineering Chemistry, at pages 163-165 (Feb. 1938), also offers data on fluorine removal from drinking water. It has been found that the technology dealing with removal of fluorine from drinking water by using alkaline earth metal phosphates accomplishes, after certain modifications have been made, a more efficient removal of fluorine from phosphoric acid.